Every formula on this site comes from published, peer-reviewed work. This page lists exactly what's used, where it's from, and where the estimates have limits — so you can check the math rather than trust it.
Body mass index is weight (kg) divided by height (m) squared. Adult categories follow the World Health Organization cut-offs5: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 and above obese. BMI is a population screen, not a measure of body fat or individual health.
The default is the Mifflin-St Jeor equation1, chosen because a systematic review found it the most accurate predictive equation for resting metabolic rate in healthy adults without a body-fat measurement.3 The Revised Harris-Benedict equation2 is offered for comparison. Katch-McArdle4 is available when you supply a body-fat percentage, since it is based on lean body mass and can be more accurate for atypical body composition.
Total daily energy expenditure is BMR multiplied by a standard activity factor (1.2 sedentary to 1.9 very active). The component breakdown splits TDEE into BMR, non-exercise activity (NEAT), exercise activity (EAT), and the thermic effect of food (TEF). TEF is estimated from macronutrients — protein has the highest thermic effect, then carbohydrate, then fat.6 The NEAT/EAT split is an illustrative estimate, not a measurement.
Weight-loss and gain targets apply a deficit or surplus to TDEE, using the approximation of roughly 7,700 kcal per kilogram of body weight. This rule is a useful planning heuristic, not a fixed law — real energy balance is more dynamic.8 The planner recomputes BMR and TDEE every week as your weight changes, and never sets a weight-loss target below your BMR.
The planner can optionally model adaptive thermogenesis — the additional fall in metabolic rate during sustained dieting, beyond what weight loss alone predicts.7 We cap this at a modest 8% and ramp it in over weeks, because the effect is real but commonly overstated; the evidence puts it in the range of roughly 100–150 kcal for a typical dieter, not enough to erase a sensible deficit.
Every output here is an estimate. Predictive equations approximate; individual metabolism varies with genetics, hormones, sleep, and measurement error in how accurately food and activity are tracked. Use these numbers as a starting point, track what actually happens, and adjust — which is exactly what the tracker is for. None of this is medical advice.